var reverseList = function(head) {
    let prev = null
    let cur = head
    while(  cur ){
        const next = cur.next  //把当前节点的next存起来
        cur.next = prev      //让当前节点的next = prev
        prev = cur           //prev = 当前节点
        cur = next           //当前节点往后移动
    }
    return prev
};